Golf club head

ABSTRACT

A golf club head is provided that includes a face part formed of a metallic material. The face part includes a thick-walled portion centrally located in the face part and configured as an elliptical or circular shape having depressions on two opposite sides thereof, and an outer peripheral portion located on the outer periphery thereof. The thick-walled portion includes a central portion having a first thickness which is greatest in the face part; a first ridge surrounding the central portion and having a second thickness less than the first thickness; and a first valley located between the central portion and the first ridge and having a third thickness less than the second thickness. The outer peripheral portion includes two first areas respectively adjacent to the two depressions of the thick-walled portion and having a fourth thickness less than the third thickness.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2010-273449 filed Dec. 8, 2010, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a golf club head, and more particularly, to a golf club head including a face member of which a thick-walled portion on the back surface has a varying thickness with respect to positions thereof.

Most recent wood club heads are configured so that at least a face part is formed of a metallic material. The wall thickness of the face part must be increased so as to maintain a strength capable of withstanding a shock impacted by a ball. The head size has been continually increased and, on the other hand, the golf rule stipulates that the volume of a golf club head must be less than 460 cm³ plus an allowance of 10 cm³. Therefore, the head volumes of most drivers are very close to the upper limit of 460 cm³. When the head size is increased, the sweet area expands, the peripheral weight distribution is emphasized, and the transverse and vertical moments of inertia increase, so that errors at the time of off-center hitting can be alleviated. However, if the head size is increased and thereby the head weight is also increased, the swing balance is intensified and the head speed drops, by which the carry may be decreased. To solve this problem, the following measures have been taken: forming the whole head of titanium or an alloy thereof (unless otherwise mentioned, hereinafter, “titanium or an alloy thereof” is referred to as “titanium”) having a small specific gravity and a high strength, or forming a composite head of carbon and titanium.

Also, many types of highly resilient heads having not only a large head size, but also a high restitution coefficient of the head, have been developed. From the year 2008, a highly resilient head having a restitution coefficient of 0.830 or more cannot be used in competition. So far, a thick-walled face material having an intensified restitution coefficient has been used positively along with the increase in head size. However, even if a highly resilient head is used, when a ball is hit by another face part than the sweet area, that is, at the time of off-center hitting, a spring effect cannot be anticipated, and the carry tends to decrease extremely.

To solve such a problem, U.S. Pat. No. 6,824,475 discloses a golf club head which is provided with a thick-walled portion surrounding the center of the back surface of the face member so as to decrease the thickness at the center of the face. As a result, when a ball is hit at a position off the sweet spot, a high initial velocity can be imparted to the ball.

In addition, U.S. Pat. No. 7,448,961 discloses a golf club head which is provided with a protruded block in the center of the back surface of a face and two annular ribs so as to surround this protruded block, thereby intensifying the strength of the golf club head and further enabling a stabilized hitting.

SUMMARY OF THE INVENTION

However, in the golf club heads disclosed in the patent documents, because the back surface of the face is provided with an increased thickness, the weight of an entire head is increased as compared to a case in which the back surface is provided with no increased thickness. Furthermore, there is another problem in that a contact time value (hereinafter called a “CT value”), which shows the resilience property of a ball, may vary depending on a hitting point of the ball.

In views of the problems mentioned above, an object of the present invention is to provide a golf club head maintaining a strength of a face member and achieving a reduction of the weight of the face member while suppressing dispersion of CT values of the face member so that the CT values at other hitting points than a sweet spot are substantially equal to the CT value at the sweet spot.

To achieve the object, the present invention provides a golf club head having a face part formed of a metallic material, the face part including: (A) a thick-walled portion centrally located in the face part and configured as a substantially elliptical or circular shape having substantially arc-shaped depressions on two opposite sides thereof, each of the depressions being directed toward a center of the face part, wherein the thick-walled portion including: (a) a central portion having a first thickness which is greatest in the face part; (b) a first ridge surrounding the central portion and having a second thickness less than the first thickness; and (c) a first valley located between the central portion and the first ridge and having a third thickness less than the second thickness and; (B) an outer peripheral portion located on an outer periphery of the thick-walled portion, wherein the outer peripheral portion including: (a) two first areas respectively adjacent to the two substantially arc-shaped depressions of the thick-walled portion and having a fourth thickness less than the third thickness; and (b) two second areas respectively extending from the substantially elliptical or circular line of the thick-walled portion toward a toe side of a sole of the head and a heel side of a crown of the head and having a thickness greater than the fourth thickness.

The two first areas of the outer peripheral portion may be extended from the two substantially arc-shaped depressions of the thick-walled portion up to ends of the face part. The two second areas of the outer peripheral portion may be extended from the substantially elliptical or circular line of the thick-walled portion up to ends of the face part. One of the two first areas of the outer peripheral portion may be adjacent to one of the second areas of the outer peripheral portion.

The two first areas of the outer peripheral portion may have a thickness of 1.8 to 2.2 mm. The two second areas of the outer peripheral portion may have a thickness of 2.5 to 3.0 mm. The outer peripheral portion may further comprise a third area having a medium thickness between the first areas and the second areas.

An axis of the substantially elliptical or circular shape of the thick-walled portion that passes through both centers of the two substantially arc-shaped depressions may be inclined with respect to a horizontal line extending the face part, the axis shifting to a crown side on a toe side of the head and to a sole side on a heel side of the head. The first valley may have a length between an upper side and a lower side thereof which is about 30% to about 50% of a height of the face part passing a center thereof. The first ridge may have a length between an upper side and a lower side thereof which is about 40% to about 60% of a height of the face part passing a center thereof.

A difference between the thicknesses of the central portion and the first ridge may be 0.05 mm to 0.5 mm. A difference between the thicknesses of the first ridge and the first valley may be 0.05 mm to 0.3 mm. The thick-walled portion may further include: a second ridge surrounding the first ridge and having a thickness less than the thickness of the first ridge; and a second valley located between the first and second ridges and having a thickness less than the thickness of the second ridge.

Consequently, according to the golf club head in accordance with the present invention, the weight of the face part can be reduced without decreasing the strength of the face part. In addition, dispersion of CT values at respective hitting points can be suppressed, and thus, the CT values of other hitting points than the sweet spot become substantially the same as the CT value of the sweet spot. As a result, if a player hits a ball off the sweet spot, the carry of the ball can increase.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view showing an embodiment of a golf club head according to the present invention;

FIG. 2 is a schematic cross-sectional view of the golf club head taken along the line II-II of FIG. 1;

FIG. 3 is a back surface view of a face part constituting the golf club head shown in FIG. 1.

FIG. 4 is a schematic cross-sectional view of the face part taken along the line III-III of FIG. 3;

FIG. 5 is a back surface view showing the relationships the heights of a central portion, a first valley and a first ridge with respect to the height of the face part shown in FIG. 3;

FIG. 6 is a back surface view showing the angle of an ellipse in the face part shown in FIG. 3;

FIG. 7 is a back surface view showing a face part according to a second embodiment;

FIG. 8 is a schematic cross-sectional view showing a face part according to a third embodiment;

FIG. 9 is a schematic cross-sectional view showing a face part according to a fourth embodiment;

FIG. 10 is a back surface view showing a face part according to a fifth embodiment;

FIG. 11 is a back surface view showing the positions of hitting points in simulation of examples; and

FIG. 12 is a back surface view of a face part according to a comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a golf club head according to the present invention will now be described with reference to the accompanying drawings. In the meantime, the drawings are intended to make the configuration of the present invention understood easily and are not represented to scale.

As shown in FIGS. 1 and 2, a golf club head 1 in this embodiment is constituted of a face member 10 having a face part and a main body member 30. The main body member 30 includes a crown part 4, a sole part 5, a hosel part 6, and a side part 7 and those parts are formed integrally. This face member 10 and the main body member 30 are joined together by welding so as to construct a golf club head 1 having a hollow structure inside. In FIG. 1, constructions appearing on a hollow-structure side surface, i.e., a back surface, of the face member 10 are indicated by broken lines. On the back surface of the face member 10, irregularities for altering the thickness of the face member 10 are formed. The back surface of the face member 10 is explained.

As shown in FIG. 3, on the back surface of the face member 10, a thick-walled portion 16 is formed within an ellipse indicated by a line 13 along an outer peripheral portion 11 essentially having a flat surface. At both end portions of the major axis of the ellipse, depressions 15 a, 15 b are formed so that the thick-walled portion 16 is depressed from the opposite ends toward the center thereof by the depressions each having an arc shape. That is, the thick-walled portion 16 is surrounded by the depressions 15 a, 15 b and the elliptical lines 13 on both sides of the minor axis of the ellipse.

As shown in FIGS. 3 and 4, in the center of the thick-walled portion 16, there is formed a central portion 17, of which the surface is flat and has a substantially trapezoidal shape and which has the largest thickness T₁ in the face member 10. The thickness of the face member 10 decreases gradually from this central portion 17 toward the elliptical line 13 in radial directions at first and then increases gradually. That is, a first valley 19 is formed to surround the central portion 17.

The thickness of the face member 10 increases gradually from the first valley 19 toward the elliptical line 13 in radial directions and then decreases gradually again. That is, a first ridge 18 is formed to surround the first valley 19.

The thickness of the face member 10 decreases gradually from the first ridge 18 toward the elliptical line 13 or the depressions 15 in radial directions. The thickness at the outer edge of the thick-walled portion 16, i.e., the thickness at the elliptical line 13 or at the depressions 15 is basically equal except as portions which will be mentioned below.

In a region extending from the elliptical line 13 or the depressions 15 to the outer edge of the face member 10, i.e., an outer peripheral portion 11, the thickness of the face member 10 does not gradually change in radial directions as described above. The outer peripheral portion 11 is divided into three types of areas having different thicknesses from each other, but each of the thicknesses is substantially uniform in the area itself. A first area is a small-thickness area 14 having a smaller thickness, which is surrounded by the depression 15 and the end portion of the elliptical line 13 at the major axis of the ellipse. A second area is a large-thickness area 12 having a greater thickness, which extends from the elliptical line 13 of the thick-walled portion 16 toward a heel side on the crown or a toe side on the sole up to the outer edge of the face member 10. A third area, which is an area excluding the first and second areas, is a medium-thickness area 11 a having a medium thickness between the small-thickness area 14 and the large-thickness area 12. The configurations of the thick-walled portion 16 and the outer peripheral portion 11 will be described in more detail.

The central portion 17 of the thick-walled portion 16 includes a sweet spot of the golf club head 1. In addition, the central portion 17 includes an intersection of the major and minor axes of the ellipse 13, i.e., a center of the ellipse. This intersection may be the same as or different from the sweet spot.

The thickness T₁ of the central portion 17 is preferably in a range of 2.0 mm to 3.8 mm. The thickness T₂ of the first ridge 18 is smaller than the thickness T₁ of the central portion 17 and is preferably in a range of 2.0 mm to 3.6 mm. A difference between these thicknesses T₁ and T₂ is preferably at least 0.05 mm, and more preferably at least 0.1 mm. This difference is preferably at most 0.5 mm, and more preferably at most 0.4 mm.

A thickness T₃ of the first valley 19 is smaller than the thickness T₂ of the first ridge 18 and is preferably in a range of 2.0 to 3.5 mm. A difference between these thicknesses T₂ and T₃ is preferably at least 0.05 mm, and more preferably at least 0.2 mm. This difference is preferably at most 0.3 mm, and more preferably at most 0.2 mm.

A thickness T₅ of the medium-thickness area 11 a of the outer peripheral portion 11 is smaller than the thickness T₃ of the first valley 19 and preferably in a range of 2.2 to 2.5 mm. A difference between these thicknesses T₃ and T₅ is preferably at least 0.1 mm, and more preferably at least 0.3 mm. This difference is preferably at most 1.5 mm, and more preferably at most 1.0 mm.

A thickness T₄ of the small-thickness area 14 of the outer peripheral portion 11 is preferably in a range of 1.8 to 2.2 mm. A difference between the thicknesses T₄ and T₅ is preferably at least 0.1 mm, and more preferably at least 0.2 mm. The thickness of the large-thickness area 12 of the outer peripheral portion 11 is smaller than the thickness T₂ of the first ridge 18 and is preferably in a range of 2.5 to 3.0 mm. A difference between the thicknesses of the large-thickness area 12 and the medium-thickness area 11 a is preferably at least 0.5 mm, and more preferably at least 0.7 mm. The thickness of the thick-walled portion 16 gradually decreases from the first ridge 18 to the large-thickness area 12 of the outer peripheral portion 11. That is, the thickness along the elliptical line 13 of the thick-walled portion 16 is different between a portion adjacent to the large-thickness area 12 of the outer peripheral portion 11 and other portions on the elliptical line 13.

The large-thickness area 12 of the outer peripheral portion 11 is formed so that the width thereof decreases gradually from a position adjacent to the thick-walled portion 16 up to an end of the face member 10. A lower limit of the width of the large-thickness area 12 is preferably at least 2 mm, and more preferably at least 3 mm. An upper limit of the width of the large-thickness area 12 is preferably at most 15 mm, and more preferably at most 12 mm.

As shown in FIG. 5, when a height H₁ of the face member 10 passing the center of the face member 10 is 100%, a length H₂ between the upper and lower sides of the first ridge 18 of the thick-walled portion 16 in the height direction of the face member 10 is preferably in a range of 40 to 60%. Likewise, a length H₃ between the upper and lower sides of the first valley 19 is preferably in a range of 30 to 50%. Furthermore, a length H₄ between the upper and lower sides of the central portion 17 is preferably in a range of 10 to 40%.

As shown in FIG. 6, the major axis of the ellipse represented by the elliptical line 13 is inclined so that the toe 3 side thereof shifts to the crown side and the heel 2 side thereof shifts to the sole side. This inclination is designed to correspond to a fact that generally, variations in hitting points of golfers are generally biased to the crown side on the toe 3 side and to the sole side on the heel 2 side. Specifically, when a golf club head is placed at an ordinary address position, the ellipse is arranged such that the inclination θ of the major axis 42 thereof is preferably at least about 5° with respect to a horizontal line 40, and more preferably at least about 10°. The inclination θ of the major axis 42 of the ellipse is preferably at most about 40°, and more preferably at most about 30°.

A ratio of the lengths between the major axis and the minor axis of the ellipse shown by the elliptical line 13 is preferably in a range of 100:50 to 50:50, and more preferably in a range of 95:50 to 70:50 (when the major axis and the minor axis are equal in length, not an ellipse but a circle is formed). A ratio between the length of the thick-walled portion 16 along the major axis of the ellipse (that is, a length between the depressions 15 a and 15 b) and the length on the minor axis of the ellipse is preferably in a range of 5:4 to 5:6. A curvature radius of the depression 15 is preferably at least about 12 mm, and more preferably at least about 13 mm. The curvature radius of the depression 15 is preferably at most about 25 mm, and more preferably at most about 20 mm.

The face member 10 may be formed either by forging or casting. As the material of the face member 10, titanium, titanium alloy or stainless steel may be used. The volume of the golf club head 1 is preferably at least about 160 cc, and more preferably at least about 350 cc such as a large head. The volume of the golf club head 1 is preferably at most about 500 cc, and more preferably at most about 480 cc. Furthermore, the weight of the golf club head 1 is preferably at least about 150 g, and more preferably at least about 160 g. The weight of the golf club head 1 is preferably at most about 250 g, and more preferably at most about 200 g such as a light head.

For ease of understanding the configuration of the present invention, FIG. 4 shows the face member 10 so that the ball hitting surface thereof is flat, but the ball hitting surface of the face member 10 may be formed with a bulge having a curvature radius of 250 to 800 mm. Also, similarly, the ball hitting surface of the face member 10 may be formed with a roll having a curvature radius of 250 to 800 mm.

The embodiment of the present invention has been described with reference to FIGS. 1 to 6, but the present invention is not limited to this. For example, as shown in FIG. 3, the small-thickness areas 14 a, 14 b in the outer peripheral portion 11 are formed between the depressions 15 a and 15 b and the elliptical line 13, but, as shown in FIG. 7, small-thickness areas 14 c, 14 d in the outer peripheral portion 11 may be extended up to ends of the face member 10 on the toe side and the heel side from the depressions 15 a, 15 b.

In addition, as shown in FIG. 3, the large-thickness portions 12 a, 12 b in the outer peripheral portion 11 are formed so that the width thereof decreases gradually from the thick-walled portion 16 toward the outer periphery, but, as shown in FIG. 7, large-thickness areas 12 c, 12 d in the outer peripheral portion 11 may be formed so that the width thereof increases gradually from the thick-walled portion 16 toward the outer periphery. Also, as shown in FIG. 7, the large-thickness areas 12 c, 12 d in the outer peripheral portion 11 may be located adjacent to the small-thickness areas 14 c, 14 d. As a result, medium-thickness portions 11 c, 11 d in the outer peripheral portion 11 are located between the top end of the thick-walled portion 16 and the crown-side edge of the face member 10 and between the bottom end of the thick-walled portion 16 and the sole-side edge of the face member 10. The expansion of the areas of the small-thickness areas 14 in the outer peripheral portion 11 is able to achieve a further reduction of the weight of the face member 10.

Furthermore, as shown in FIG. 4, the thick-walled portion 16 is formed so that the thickness thereof is changed gradually, but the present invention is not limited to this. As shown in FIG. 8, the thick-walled portion 16 may be formed so that the bottom of the first valley 19 has a specific area having the thickness T₃ and also the top of the first ridge 18 has a specific area having the thickness T₂. Even when the thickness of the thick-walled portion 16 is changed in a staircase fashion, the same effect can be obtained.

According to another embodiment, the first valley 19 may be located just below an edge of the central portion 17 instead of the thickness being decreased gradually from the edge of the central portion 17 to the first valley 19 as shown in FIG. 9. Likewise, the first ridge 18 may be located just above the depression 15 instead of the width being decreased gradually from the first ridge 18 to the depression 15 as shown in FIG. 9. The same effect may be obtained by changing the thickness of the thick-walled portion 16.

Although in FIG. 3, a valley and a ridge are formed in the thick-walled portion 16 as a pair, the present invention is not limited to this example, and plural pairs of the valleys and ridges may be formed. For example, as shown in FIG. 10, a second valley 22 and a second ridge 21 are formed in order so as to surround the first ridge 18.

The thickness of the second valley 22 is less than the thickness T₃ of the first valley 19 and at the same time, greater than or equal to the thickness T₅ of the medium-thickness area 11 a. Preferably, a difference between the thickness of the second valley 22 and the thickness T₃ of the first valley 19 is 0.1 mm or more, and more preferably, 0.3 mm or more. Furthermore, this difference is preferably 1.0 mm or more and more preferably 0.8 mm or less.

The thickness of the second ridge 21 is less than the thickness T₂ of the first ridge 18 and greater than the thickness of the second valley 22. Preferably, a difference between the thickness of the second ridge 21 and the thickness T₂ of the first ridge 18 is 0.1 mm or more, and more preferably, 0.3 mm or more. This difference is preferably 1.5 mm or less and more preferably 1.0 mm or less.

The embodiment in which the entire face part of the golf club head 1 serves as the face member 10 as shown in FIG. 1 has been described above. However, the present invention is not limited to this embodiment, and it is possible to form a central portion of the face part as the face member and form other portions on both ends of the face part integrally with the head main body.

EXAMPLES

CT values of a face member according to the present invention were calculated through a computer simulation (Example 1). The face member had the structural features shown in FIGS. 3 and 4. Detailed conditions of the face member are below. The thickness T₁ of the central portion of the thick-walled portion was set to 3.7 mm. The thickness T₂ of the first ridge was set to 3.5 mm. The thickness T₃ of the first valley was set to 3.0 mm. In the outer peripheral portion, the thickness of the large-thickness areas was set to 2.7 mm, the thicknesses of the small-thickness areas on the toe side and the heel side were set to 2.2 mm and 1.9 mm, respectively, and the thickness T₅ of the medium-thickness area was set to 2.4 mm. The height H₁ of the face member was set to 50 mm. The length H₂ between the upper side and the lower side of the first ridge was set to 30 mm. The length H₃ between the upper side and the lower side of the first valley was set to 20 mm. The length H₄ between the upper side and the lower side of the edge of the central portion 17 was set to 2.4 mm.

The lengths of the major and minor axes of the ellipse were set to 64 mm and 40 mm, respectively. The angle θ of the ellipse was set to 15°. The curvature radiuses of the depressions on the toe side and the heel side were set to 32 mm and 12 mm, respectively. The ratio of areas between the outer peripheral portion and the thick-walled portion of the face part was set to 7:5. This ratio was calculated with the central portion included in the thick-walled portion, and the area of the central portion was about 3% of the entire face part. The width of the large-thickness area of the outer peripheral portion was set to 4 to 10 mm. The face member was supposed to be made of titanium alloy, so that the Young modulus and the Poisson ratio thereof were 108 GPa and 0.30, respectively.

As shown in FIGS. 11 and 15, different hitting points in a vertical direction and a horizontal direction were investigated. A hitting point 50 c in the center was assumed to be a hitting point located on the sweet spot and within the central portion of the face member. Fourteen other hitting points were assumed to be hitting points apart from the sweet spot. The hitting points 10 mm apart from the hitting point 50 c in the center to the toe side and the heel side were designated as hitting points 50 b and 50 d, the hitting points 10 mm further apart to the toe side and the heel side were designated as hitting points 50 a and 50 e. The hitting points 10 mm apart from these five hitting points 50 a to 50 e to the crown side and the sole side were designated as hitting points 50 aH to 50 eH and 50 aL to 50 eL.

The hitting points 50 a, 50 aH, 50 aL, 50 e, 50 eH were located substantially between the ridge of the thick-walled portion and the depression or the elliptical line. The hitting point 50 eL was located in the small-thickness area of the outer peripheral portion. The hitting points 50 b, 50 bH, 50 cH, 50 cL, 50 d, 50 dL were located substantially between the edge of the central portion and the first valley. The hitting points 50 bL, 50 dH were located between the first valley and the first ridge.

For comparison, the same simulation as described above was performed using a face member having a thick-walled portion and ribs as shown in FIG. 12 (Comparative Example 1). The thickness of a central portion 67 is 3.7 mm at most like in Example 1 and the thick-walled portion 66 was so constructed that the thickness thereof decreased gradually in a radial manner from the central portion toward the outer periphery of the thick-walled portion 66. The thickness of the outer periphery of the thick-walled portion 66, i.e., the thickness of the outer peripheral portion 11 was set to 2.4 mm like in Example 1. The thickness of the small-thickness area of an outer peripheral portion 64 was set to 2.1 mm both on the toe side and the heel side. According to Comparative Example 1, two ribs extending from the central portion 67 to the outer periphery of the face member were provided. The thickness of the ribs 62 was set to 3.3 mm.

Tables 1 and 2 show the simulation results of Example 1 and Comparative Example 1. According to these tables, with the position of the hitting point 50 c in the center assumed as 0 mm, the distance directed to the crown side or the heel side was expressed with plus and the distance directed to the sole side or the toe side was expressed with minus. The results of the tables were expressed with a ratio assuming the CT value of the hitting point 50 c in the center was assumed as 100.

TABLE 1 CT value (ratio) Example 1 −20 mm −10 mm 0 mm 10 mm 20 mm 10 mm 102  96  95 100 102  0 mm 101 101 100 101 101 −10 mm    88  94  94  92  85

TABLE 2 Comparative CT value (ratio) example 1 −20 mm −10 mm 0 mm 10 mm 20 mm 10 mm 99 97 95 98 102  0 mm 98 101  100  102  103 −10 mm   85 91 95 91  87

Compared to Comparative example 1, in which the thickness of the thick-walled portion was decreased gradually from the central portion toward an outside and the ribs were provided extending from the central portion up to the outer periphery, Example 1, in which the thickness was changed by forming valleys and ridges in the thick-walled portion and the large-thickness area was provided on the outer peripheral portion, could obtain uniform CT values, particularly, at hitting points 0 mm and 10 mm in height. This indicates that even when a ball is hit off the sweet spot, a similar resilience property to when hitting the ball on the sweet spot can be exerted.

While the weight of the face member of Comparative example 1 was 31.99 g, the weight of the face member of Example 1 was 31.37 g. That is, the present invention has attained a reduction in weight of 0.62 g. 

1. A golf club head comprising a face part formed of a metallic material, the face part comprising: a thick-walled portion centrally located in the face part and configured as a substantially elliptical or circular shape having substantially arc-shaped depressions on two opposite sides thereof, each of the depressions being directed toward a center of the face part, wherein the thick-walled portion comprises: a central portion having a first thickness which is greatest in the face part; a first ridge surrounding the central portion and having a second thickness less than the first thickness; and a first valley located between the central portion and the first ridge and having a third thickness less than the second thickness and; an outer peripheral portion located on an outer periphery of the thick-walled portion, wherein the outer peripheral portion comprises: two first areas respectively adjacent to the two substantially arc-shaped depressions of the thick-walled portion and having a fourth thickness less than the third thickness; and two second areas respectively extending from the substantially elliptical or circular line of the thick-walled portion toward a toe side of a sole of the head and a heel side of a crown of the head and having a thickness greater than the fourth thickness.
 2. The golf club head according to claim 1, wherein the two first areas of the outer peripheral portion are extended from the two substantially arc-shaped depressions of the thick-walled portion up to ends of the face part.
 3. The golf club head according to claim 1, wherein the two second areas of the outer peripheral portion are extended from the substantially elliptical or circular line of the thick-walled portion up to ends of the face part.
 4. The golf club head according to claim 1, wherein one of the two first areas of the outer peripheral portion is adjacent to one of the second areas of the outer peripheral portion.
 5. The golf club head according to claim 1, wherein the two first areas of the outer peripheral portion have a thickness of 1.8 to 2.2 mm and the two second areas of the outer peripheral portion have a thickness of 2.5 to 3.0 mm.
 6. The golf club head according to claim 1, wherein the outer peripheral portion further comprises a third area having a medium thickness between the first areas and the second areas.
 7. The golf club head according to claim 1, wherein an axis of the substantially elliptical or circular shape of the thick-walled portion that passes through both centers of the two substantially arc-shaped depressions is inclined with respect to a horizontal line extending to the face part, the axis shifting to a crown side on a toe side of the head and to a sole side on a heel side of the head.
 8. The golf club head according to claim 1, wherein the first valley has a length between an upper side and a lower side thereof which is about 30% to about 50% of a height of the face part passing a center thereof.
 9. The golf club head according to claim 1, wherein the first ridge has a length between an upper side and a lower side thereof which is about 40% to about 60% of a height of the face part passing a center thereof.
 10. The golf club according to claim 1, wherein a difference between the thicknesses of the central portion and the first ridge is 0.05 mm to 0.5 mm and a difference between the thicknesses of the first ridge and the first valley is 0.05 mm to 0.3 mm.
 11. The golf club head according to claim 1, wherein the thick-walled portion further comprises: a second ridge surrounding the first ridge and having a thickness less than the thickness of the first ridge; and a second valley located between the first and second ridges and having a thickness less than the thickness of the second ridge. 